The present invention relates to fasteners, and more particularly to a tamper resistant threaded fastener for composite applications, as well as a fastener-tool system for achieving a tamper resistant threaded connection.
Prior known devices for achieving a tamper resistant threaded connection have included threaded fasteners having torque-limiting tool engaging portions designed to fail in shear and separate from a threaded body to leave a generally smoothly contoured threaded connection to which it is difficult to apply a torque for removing the fastener. However, such fasteners possess the disadvantage of requiring costly close tolerance machining operations for achieving generally complicated configurations which accomplish the desired result. Moreover, while the resulting smoothly contoured assembly of such devices is well suited to prevent the application of a torque for backing-off the fastener once it has been set, such an assembly inhibits or prevents a re-torquing of the fastener if necessary. The provision for re-torquing is highly desirable when sealants or adhesives are utilized with threaded fasteners. Such compositions can cause a change in stack dimensions due to curing or bleed-out, a situation which occurs frequently in composite applications. When this condition occurs, a re-torquing of the fastener is often advisable, if not imperative.
It is therefore desirable to provide a tamper resistant threaded fastener of a relatively simple configuration which can be manufactured using simpler and less expensive methods, as well as a tool for driving such a fastener. It is moreover desirable to provide such a tamper resistant fastener which maintains its wrenching capability after installation so that it can be re-torqued if necessary.
The tamper resistant threaded fastener of the present invention includes an internally threaded body portion for receiving a threaded bolt, and a hollow cylindrical driving portion spaced radially outward from and interconnected with the body portion by a radial flange. The driving portion includes a serrated driving surface which defines a plurality of recesses spaced circumferentially about the driving portion and adapted to receivably engage a like number of generally complementary shaped projections on a driving tool. These recesses and projections cooperate to establish an operative connection between the driving portion and the driving tool when the tool is rotated in a direction for advancing and setting the fastener. However, the recesses and projections cooperate to eject the projections from the recesses and break this connection if the tool is rotated in a direction for backing-off the fastener once it has been set. The fastener can be manufactured in a simple stamping operation and therefore can be made more cheaply than prior known devices. It therefore provides a simplified, less expensive device for achieving a tamper resistant threaded connection. The fastener is particularly suited for use in composite applications, since it provides an assembled configuration which, while inhibiting the application of a back-off torque to the assembly, allows a user to apply additional torque for resetting the fastener as desired.
The above and other features of the invention will become apparent from a reading of the detailed description of the preferred embodiments which make reference to the following set of drawings, in which: